The present invention relates to an improved device for sensing the level of liquid, particularly for submersed pumps.
As is known, submersed pumps are applied most frequently in evacuating traps or the like, to remove an excessive quantity of liquids gradually accumulated therein.
A typical case is the emptying of drainage traps and of water accumulated in excavations in building yards.
Submersed pumps usually have devices for sensing the level of liquid that allow actuation when the level of the liquid exceeds a maximum value and stop operation if the level of liquid reaches a minimum value.
These known sensing devices are substantially constituted by appendages that protrude outside the dimensions of the pump with end floats which, by being affected by the rise or fall of the level of liquid, activate electrical connections.
Since they protrude outside the overall dimensions of the pumps, known sensing devices require the traps or spaces from which the liquid is to be evacuated to be larger than such overall dimensions of the pump.
Moreover, the correct operation of these devices depends also on whether floating objects are present or not within the liquid.
The presence of a floating foreign object might in fact alter the readings of the device or even damage it.
Sometimes the readings of the devices can also be biased as a consequence of a turbulent motion of the liquid.
To solve these drawbacks, a sensing device has been devised which is the subject of U.S. patent application Ser. No. 09/959,343 in the name of this same Applicant and comprises means for sensing the variation in the electrical and/or magnetic field in relation to the variation of the level of the liquid.
An embodiment of these sensing means is constituted by two pairs of plates, arranged one above the other, at a pipe in which the liquid is present.
The plates arranged in an upward region signal that the upper level of the liquid has been reached to a control unit, which switches on the pump.
The plates arranged in a downward region instead signal when it is timely to stop the pump.
To detect the variation in the capacitance of the capacitors that produces the level signal, the method of measuring their charging time is used.
The capacitance of each capacitor varies considerably when water or air is present as dielectric between the plates (the difference is approximately two orders of magnitude), and the charging time is affected thereby importantly, although the capacitances to be measured are small in absolute terms.
FIG. 1 of the accompanying drawings plots the charging and discharging time of a capacitor.
The dashed line relates to the capacitor with air as the dielectric, while the solid line relates to water as the dielectric.
The considerable difference between the charging times t1 and t2 and the discharging times t3 and t4 is evident.
Electronics currently provides very fast logic units or microcontrollers, and the measurement is performed easily.
Twin-capacitor devices provide a series of advantages:
two distinct and clearly identified capacitances are measured in order to check the maximum and minimum levels, because all metallic plates are used and no parasitic capacitors that would introduce uncertainties are used;
the sensing of the variation in the capacitance according to the charging and/or discharging time does not depend on environmental factors but on a basic principle of physics, the capacitances vary to an important extent, and it is necessary to distinguish only two values per sensor (water or air), digitizing the signal;
with digital circuitry there is no drift due to analog components.
Practical execution nonetheless has revealed drawbacks, in particular the need to provide recesses, irregular shapes, insertion of pipes, in order to avoid problems due to dirt, floating bodies, vortical motions and clogging.
The aim of the present invention is to further improve devices for sensing the level of liquid based on capacitors by eliminating the problems of known types noted above.
Within this aim, an object of the invention is to provide a device for sensing the level of liquid whose sensing cannot be influenced by any turbulent motions of the liquid or by the presence of foreign objects floating within said liquid.
Another object is to provide a sensing device that is contained within the dimensions of the pump.
Another object is to be able to provide pumps that are more compact than current ones.
Another object is to provide a device that gives advantages in terms of production both to the pump and to the control and actuation circuits.
This aim and these and other objects that will become better apparent hereinafter are achieved by an improved device for sensing the level of liquid, particularly for submersed pumps, of the type that comprises capacitors with two plates arranged one above the other, characterized in that said plates are arranged in vertical succession at different levels.
Advantageously, said capacitors are two and are formed by three plates arranged vertically on three different levels, the intermediate plate being shared by the two capacitors and being connected to the ground.